1. Field of the Invention
The present invention relates to a flat cable used in, for example, electronic devices.
2. Description of the Background Arts
Size and weight reduction of electronic devices has lead to the miniaturization of the electronic components and wiring members installed in these devices. Wiring members are in demand that can be wired densely in limited space. Examples of such wiring members include flexible printed circuit boards, flat cables that use flat conductors, and electrical connectors used to connect the printed circuit boards or flat cables. With these wiring members, in which a large number of electrical conductors are densely arranged, the conductors are electrically insulated from each other and satisfactory electrical connections are required.
The electrical conductors are normally made of copper, which has good electrical conductivity, good ductility, and high strength, and is easily coated with other metals. For the wiring members that use copper, tin plating is commonly used for its corrosion resistance and soldering performance. Tin plating is normally formed by electroplating, and the formation of acicular crystal structures (hereinafter referred to as whiskers) on the surface of the tin plating is a known phenomenon.
When a copper-based metal material is plated with tin, the copper atoms are diffused throughout the tin plating film to create a copper-tin intermetallic compound. This intermetallic compound has a different crystal structure from tin, and strain is produced in the crystal lattice such that compressive stress is created in the tin plating film. This compressive stress acts as a drive force for whisker growth, and whiskers are therefore believed to form easily in cases in which a copper-based material is plated with tin. Whiskers are a cause of electrical short-circuiting between conductors, and various improvement measures have therefore been proposed.
Japanese Patent Application Laid-Open No. 2001-43743 discloses a flat cable that comprises a flat conductor plated with a tin-copper alloy to inhibit the formation of whiskers in the plating later. Japanese Patent Application Laid-Open No. 10-46385 discloses an electric/electronic circuit component that is covered with a tin-zinc alloy plating to inhibit the formation of whiskers.
As disclosed in these documents, whiskers can be inhibited to a certain extent with plating made of a tin-copper alloy or a tin-zinc alloy instead of tin alone. However, when terminal parts of flat cables are used as male contacts and connected to electrical connectors, the surface of the plating is subjected to external stress from the point of contact of the connector, whiskers are likely to form in a specific manner, and the whiskers increase in length. Therefore, in order to prevent whisker-induced short-circuiting in this portion, further measures must be taken to inhibit the formation and growth of whiskers.